Start a local 3 player match, all human players, with 5 town halls needed for a victory. Split the territories between your main account and one of the two other human players. Defeat the 3rd human player. Capture 3 or 4 of the larger districts with your main account, and the other districts with the other player.

Now keep building radio towers all across your territories, and do the same for the 2nd player. Whenever you have to place troops, place them in a territory bordering one of the other player's territories. In that specific territory, also build a defensive position facility. Whenever player 2 has troops to spend, waste all of that troops by attacking the territory with all of your troops + the defensive position. This will give you progress towards destroying 1000 enemy troops.

Since you have the most territories, you will be selected as mayor for each turn. You can choose the outlaw bounty if you need progress towards that achievement.

Once you have destroyed 1000 troops, capture all of player 2's territories, except for 2 town halls. Now just keep skipping turns until you've drafted 1000 troops. Note that only troops gathered from having territories, districts and town halls count towards the achievement. Troops bought for stars do not. I was getting 28 troops a turn, thereby making the achievement unlock pretty fast.